It has become common to use a wide area network (WAN) employing an Internet protocol-virtual private network (IP-VPN) technique or the like as a communication network between points used in a cloud or the like.
When a computer such as a terminal located at a certain point performs communication with a terminal located at another point, communication is performed through a line connecting a local area network (LAN) at its own point with a WAN and a line connecting a WAN with a LAN at another point. In the lines, an available bandwidth is limited according to a contract bandwidth.
A transmission control protocol (TCP) is commonly used in communication between terminals. In TCP communication, a receiving terminal gives feedback on a position of data that is transmitted by a transmitting terminal and received thereby to the transmitting terminal. When the fed-back position of the received data does not increase, the transmitting terminal determines that discarding has been detected.
Further, the transmitting terminal manages a parameter called a congestion window size (a transmittable data size even when reception is not notified of by the receiving terminal), and changes the congestion window size according to a round trip time (RTT) or whether discarding is detected.
When discarding is detected or when the RTT increases, a network is determined to be congested, and in this case, by reducing the window size, a transmission bandwidth is indirectly decreased, and thus congestion of the network is avoided. Further, when no discarding is detected or when the RTT decreases, the network is determined to be available, and in this case, by increasing the window size, the transmission bandwidth is indirectly increased, and a line bandwidth of the network is efficiently used.
As the RTT increases or as the discarding rate increases, the window size increases, and the bandwidth decreases.
As described above, in the communication using the TCP in the WAN, the transmission bandwidth significantly depends on the RTT and the discarding rate.
As a method of increasing the window size, there is also a technique of first non-linearly increasing the window size and then linearly increasing the window size after the window size exceeds a certain threshold value (PTL 1).
Meanwhile, PTL 2 discloses that in a communication system using the TCP in the WAN, a device connected to a receiving side terminal includes a means for giving feedback on all discarded segments to a device connected to a transmitting side terminal, and the transmitting side terminal includes a means for retransmitting the fed-back discarded segments and a means for controlling a transmission bandwidth based on a retransmission bandwidth and a discarded bandwidth.